Sensor strips of the aforedescribed type can be used for automatic control of the displacement of a door and can be provided along a closing edge thereof, to avoid endangering people or objects in the path of the closing edge and to insure that the closing edge reaches a fully closed position. In many cases the sensor strip can also fulfill a sealing function in the closed position.
Prior art sensor strips which are commercially available function with an air pressure wave generated in an elastic hose or tube. When the sensor strip provided with such a hose containing air encounters an obstruction, a deformation of the tube results in a pressure wave in the tube which is guided by the latter to a membrane switch. The switch, in turn, can control the drive for the door. Because the air pressure wave generated in this manner has a comparatively low amplitude the membrane switch must be unusually sensitive and experience has shown that such highly sensitive membrane switches generally have relatively limited life spans. The failure of the membrane switch may result in extreme danger and can cause harm and even fatal injury. As a consequence, in this field efforts have been made to provide self-monitoring or self-testing systems so that upon a failure in the function of the sensor, the entire apparatus will be switched off and dangerous conditions prevented from occurring.
In German Patent No. 2,719,438, a self-monitoring sensing strip is described which operates in accordance with the quiescent current or rest current principle and which utilizes a strip switch having at an outer end, a measuring resistor across the two conductors. As a consequence, in the rest or inactivated position, a current flow is defined by the resistance. Upon interruption of the supply line, a monitoring circuit can generate an alarm. Apart from the relatively high cost of the sensor and this equipment, there is the disadvantage that at the ends at which contact terminals are provided and where the profile is sealed to make it watertight, the device is not functional to sense an obstruction.
Furthermore, the device is directionally sensitive, i.e. is sensitive to forces applied from above in only one direction and is not sensitive to lateral forces.
In German Open Application DE-OS No. 3107847, a similar construction is provided in which electromagnetic waves, for example, light waves, are transmitted through a curved, closed hollow profile element. By detection of the light intensity at a location remote from the light force and resulting from a reduction in the cross section of the light passage because of deformation of the hollow profile, a control pulse can be provided to the drive for the window to be closed. Since the hollow profile is closed except for vent openings and is lined with reflective material, the response of this prior art construction is slow and can lead to problems.
To avoid these drawbacks a sensor strip has been developed, in the interior of which a light curtain or beam is provided in which at least in part of the light beam passes along the part of the sensor strip to be deformed upon contact with an obstruction or the end of the path of the moving member. Such deformation of the strip results in an interruption of the light beam to switch off the drive of the moving part or reverse the drive.
In the rest position or with undisturbed movement of the doors, the detector of the light beam is fully illuminated so that in this state the circuit of the drive system is not interrupted. When the door encounters an obstruction and the sensor strip is deformed, the illumination of the sensor is strongly diminished or is interrupted so that the sensor via conventional and preferably electronic circuitry, can interrupt the current supply to the drive or reverse it to thereby prevent injury or damage with a high degree of reliability.
Since the system will shut down or reverse in the event of a failure of illumination of a sensor as a result of a power failure or any other interruption in the light beam, the sensor is self-monitoring and self reliable.
A sensor strip of this type is described in German Open Application DE-OS No. 2658660.
The light beam of the light curtain in this system runs through a hollow profile with an approximately circular inner cross section. To interrupt the light path from the source to the receiver, the internal cross section of the hollow profile must at least at one location, be completely constricted.
Such a complete closure of the interior of a tubular body is not possible if that body has a relatively large wall thickness as is often required for sealing purposes or for a long useful life of the strip. Consequently, even with relatively strong inward compression of the hollow profile there maybe a substantial portion of light which reaches the receiver from the source. As a consequence, the above-described light curtain may not be completely reliable and capable of responding to all contacts with obstruction, may be excessively sensitive with respect to response to insure that a compression of the deformable member will effect the desired response, or will not switch in an entirely satisfactorily manner.
Thus in Austrian Patent No. 382,925, a patent has been described that will allow reliable interruption of a light path of the light curtain when a predetermined deformation of the sensor strip is exceeded. In this system, within the interior of the sensor strip, webs of the yieldable material with diaphragm openings are provided for passage of the light beam. As long as the light beam transverses all of the openings and the openings remain fully aligned, the sensor will receive light. With deformation of the strip one or more of the openings or webs can be disaligned with respect to others to insure an interruption.
Even this system, however, has been found to be insufficiently sensitive in practice and it has been found that, to achieve a complete interruption of the light path with this system and the systems described earlier, it has been necessary to provide a very strong deformation of the sensor strip. Even those systems which do not require complete interruption for a response generally require a strong deformation of the sensor strip to obtain the response. Indeed, systems in which a weakening of the light path is detected often have internal reflection which partly compensates for the weakening so that here as well strong deformation is necessary.